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perlglossary - Perl Glossary
A glossary of terms (technical and otherwise) used in the
Perl documentation. Other useful sources include the Free
On-Line Dictionary of Computing
<http://foldoc.doc.ic.ac.uk/foldoc/index.html>, the Jargon
File <http://catb.org/~esr/jargon/>, and Wikipedia
<http://www.wikipedia.org/>.
A
accessor methods
A "method" used to indirectly inspect or update an
"object"'s state (its instance variables).
actual arguments
The scalar values that you supply to a "function" or
"subroutine" when you call it. For instance, when you
call "power("puff")", the string "puff" is the actual
argument. See also "argument" and "formal arguments".
address operator
Some languages work directly with the memory addresses
of values, but this can be like playing with fire. Perl
provides a set of asbestos gloves for handling all
memory management. The closest to an address operator
in Perl is the backslash operator, but it gives you a
"hard reference", which is much safer than a memory
address.
algorithm
A well-defined sequence of steps, clearly enough
explained that even a computer could do them.
alias
A nickname for something, which behaves in all ways as
though you'd used the original name instead of the nick-
name. Temporary aliases are implicitly created in the
loop variable for "foreach" loops, in the $_ variable
for map or grep operators, in $a and $b during sort's
comparison function, and in each element of @_ for the
"actual arguments" of a subroutine call. Permanent
aliases are explicitly created in packages by importing
symbols or by assignment to typeglobs. Lexically scoped
aliases for package variables are explicitly created by
the our declaration.
alternatives
A list of possible choices from which you may select
only one, as in "Would you like door A, B, or C?"
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Alternatives in regular expressions are separated with a
single vertical bar: "|". Alternatives in normal Perl
expressions are separated with a double vertical bar:
"||". Logical alternatives in "Boolean" expressions are
separated with either "||" or "or".
anonymous
Used to describe a "referent" that is not directly
accessible through a named "variable". Such a referent
must be indirectly accessible through at least one "hard
reference". When the last hard reference goes away, the
anonymous referent is destroyed without pity.
architecture
The kind of computer you're working on, where one "kind"
of computer means all those computers sharing a compati-
ble machine language. Since Perl programs are (typi-
cally) simple text files, not executable images, a Perl
program is much less sensitive to the architecture it's
running on than programs in other languages, such as C,
that are compiled into machine code. See also "plat-
form" and "operating system".
argument
A piece of data supplied to a program, "subroutine",
"function", or "method" to tell it what it's supposed to
do. Also called a "parameter".
ARGV
The name of the array containing the "argument" "vector"
from the command line. If you use the empty "<>" opera-
tor, "ARGV" is the name of both the "filehandle" used to
traverse the arguments and the "scalar" containing the
name of the current input file.
arithmetical operator
A "symbol" such as "+" or "/" that tells Perl to do the
arithmetic you were supposed to learn in grade school.
array
An ordered sequence of values, stored such that you can
easily access any of the values using an integer "sub-
script" that specifies the value's "offset" in the
sequence.
array context
An archaic expression for what is more correctly
referred to as "list context".
ASCII
The American Standard Code for Information Interchange
(a 7-bit character set adequate only for poorly
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representing English text). Often used loosely to
describe the lowest 128 values of the various ISO-8859-X
character sets, a bunch of mutually incompatible 8-bit
codes best described as half ASCII. See also "Unicode".
assertion
A component of a "regular expression" that must be true
for the pattern to match but does not necessarily match
any characters itself. Often used specifically to mean a
"zero width" assertion.
assignment
An "operator" whose assigned mission in life is to
change the value of a "variable".
assignment operator
Either a regular "assignment", or a compound "operator"
composed of an ordinary assignment and some other opera-
tor, that changes the value of a variable in place, that
is, relative to its old value. For example, "$a += 2"
adds 2 to $a.
associative array
See "hash". Please.
associativity
Determines whether you do the left "operator" first or
the right "operator" first when you have "A "operator" B
"operator" C" and the two operators are of the same pre-
cedence. Operators like "+" are left associative, while
operators like "**" are right associative. See perlop
for a list of operators and their associativity.
asynchronous
Said of events or activities whose relative temporal
ordering is indeterminate because too many things are
going on at once. Hence, an asynchronous event is one
you didn't know when to expect.
atom
A "regular expression" component potentially matching a
"substring" containing one or more characters and
treated as an indivisible syntactic unit by any follow-
ing "quantifier". (Contrast with an "assertion" that
matches something of "zero width" and may not be quanti-
fied.)
atomic operation
When Democritus gave the word "atom" to the indivisible
bits of matter, he meant literally something that could
not be cut: a- (not) + tomos (cuttable). An atomic
operation is an action that can't be interrupted, not
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one forbidden in a nuclear-free zone.
attribute
A new feature that allows the declaration of variables
and subroutines with modifiers as in "sub foo : locked
method". Also, another name for an "instance variable"
of an "object".
autogeneration
A feature of "operator overloading" of objects, whereby
the behavior of certain operators can be reasonably
deduced using more fundamental operators. This assumes
that the overloaded operators will often have the same
relationships as the regular operators. See perlop.
autoincrement
To add one to something automatically, hence the name of
the "++" operator. To instead subtract one from some-
thing automatically is known as an "autodecrement".
autoload
To load on demand. (Also called "lazy" loading.)
Specifically, to call an AUTOLOAD subroutine on behalf
of an undefined subroutine.
autosplit
To split a string automatically, as the -a "switch" does
when running under -p or -n in order to emulate "awk".
(See also the AutoSplit module, which has nothing to do
with the -a switch, but a lot to do with autoloading.)
autovivification
A Greco-Roman word meaning "to bring oneself to life".
In Perl, storage locations (lvalues) spontaneously gen-
erate themselves as needed, including the creation of
any "hard reference" values to point to the next level
of storage. The assignment "$a[5][5][5][5][5] = "quin-
tet"" potentially creates five scalar storage locations,
plus four references (in the first four scalar loca-
tions) pointing to four new anonymous arrays (to hold
the last four scalar locations). But the point of auto-
vivification is that you don't have to worry about it.
AV Short for "array value", which refers to one of Perl's
internal data types that holds an "array". The "AV"
type is a subclass of "SV".
awk Descriptive editing term--short for "awkward". Also
coincidentally refers to a venerable text-processing
language from which Perl derived some of its high-level
ideas.
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B
backreference
A substring captured by a subpattern within unadorned
parentheses in a "regex". Backslashed decimal numbers
("\1", "\2", etc.) later in the same pattern refer back
to the corresponding subpattern in the current match.
Outside the pattern, the numbered variables ($1, $2,
etc.) continue to refer to these same values, as long as
the pattern was the last successful match of the current
dynamic scope.
backtracking
The practice of saying, "If I had to do it all over, I'd
do it differently," and then actually going back and
doing it all over differently. Mathematically speaking,
it's returning from an unsuccessful recursion on a tree
of possibilities. Perl backtracks when it attempts to
match patterns with a "regular expression", and its ear-
lier attempts don't pan out. See "Backtracking" in
perlre.
backward compatibility
Means you can still run your old program because we
didn't break any of the features or bugs it was relying
on.
bareword
A word sufficiently ambiguous to be deemed illegal under
use strict 'subs'. In the absence of that stricture, a
bareword is treated as if quotes were around it.
base class
A generic "object" type; that is, a "class" from which
other, more specific classes are derived genetically by
"inheritance". Also called a "superclass" by people who
respect their ancestors.
big-endian
From Swift: someone who eats eggs big end first. Also
used of computers that store the most significant "byte"
of a word at a lower byte address than the least signi-
ficant byte. Often considered superior to little-endian
machines. See also "little-endian".
binary
Having to do with numbers represented in base 2. That
means there's basically two numbers, 0 and 1. Also used
to describe a "non-text file", presumably because such a
file makes full use of all the binary bits in its bytes.
With the advent of "Unicode", this distinction, already
suspect, loses even more of its meaning.
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binary operator
An "operator" that takes two operands.
bind
To assign a specific "network address" to a "socket".
bit An integer in the range from 0 to 1, inclusive. The
smallest possible unit of information storage. An
eighth of a "byte" or of a dollar. (The term "Pieces of
Eight" comes from being able to split the old Spanish
dollar into 8 bits, each of which still counted for
money. That's why a 25-cent piece today is still "two
bits".)
bit shift
The movement of bits left or right in a computer word,
which has the effect of multiplying or dividing by a
power of 2.
bit string
A sequence of bits that is actually being thought of as
a sequence of bits, for once.
bless
In corporate life, to grant official approval to a
thing, as in, "The VP of Engineering has blessed our
WebCruncher project." Similarly in Perl, to grant offi-
cial approval to a "referent" so that it can function as
an "object", such as a WebCruncher object. See "bless"
in perlfunc.
block
What a "process" does when it has to wait for something:
"My process blocked waiting for the disk." As an unre-
lated noun, it refers to a large chunk of data, of a
size that the "operating system" likes to deal with
(normally a power of two such as 512 or 8192). Typi-
cally refers to a chunk of data that's coming from or
going to a disk file.
BLOCK
A syntactic construct consisting of a sequence of Perl
statements that is delimited by braces. The "if" and
"while" statements are defined in terms of BLOCKs, for
instance. Sometimes we also say "block" to mean a lexi-
cal scope; that is, a sequence of statements that act
like a "BLOCK", such as within an eval or a file, even
though the statements aren't delimited by braces.
block buffering
A method of making input and output efficient by passing
one "block" at a time. By default, Perl does block
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buffering to disk files. See "buffer" and "command
buffering".
Boolean
A value that is either "true" or "false".
Boolean context
A special kind of "scalar context" used in conditionals
to decide whether the "scalar value" returned by an
expression is "true" or "false". Does not evaluate as
either a string or a number. See "context".
breakpoint
A spot in your program where you've told the debugger to
stop execution so you can poke around and see whether
anything is wrong yet.
broadcast
To send a "datagram" to multiple destinations simultane-
ously.
BSD A psychoactive drug, popular in the 80s, probably
developed at U. C. Berkeley or thereabouts. Similar in
many ways to the prescription-only medication called
"System V", but infinitely more useful. (Or, at least,
more fun.) The full chemical name is "Berkeley Standard
Distribution".
bucket
A location in a "hash table" containing (potentially)
multiple entries whose keys "hash" to the same hash
value according to its hash function. (As internal pol-
icy, you don't have to worry about it, unless you're
into internals, or policy.)
buffer
A temporary holding location for data. Block buffering
means that the data is passed on to its destination
whenever the buffer is full. Line buffering means that
it's passed on whenever a complete line is received.
Command buffering means that it's passed every time you
do a print command (or equivalent). If your output is
unbuffered, the system processes it one byte at a time
without the use of a holding area. This can be rather
inefficient.
built-in
A "function" that is predefined in the language. Even
when hidden by "overriding", you can always get at a
built-in function by qualifying its name with the
"CORE::" pseudo-package.
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bundle
A group of related modules on "CPAN". (Also, sometimes
refers to a group of command-line switches grouped into
one "switch cluster".)
byte
A piece of data worth eight bits in most places.
bytecode
A pidgin-like language spoken among 'droids when they
don't wish to reveal their orientation (see "endian").
Named after some similar languages spoken (for similar
reasons) between compilers and interpreters in the late
20th century. These languages are characterized by
representing everything as a non-architecture-dependent
sequence of bytes.
C
C A language beloved by many for its inside-out "type"
definitions, inscrutable "precedence" rules, and heavy
"overloading" of the function-call mechanism. (Well,
actually, people first switched to C because they found
lowercase identifiers easier to read than upper.) Perl
is written in C, so it's not surprising that Perl bor-
rowed a few ideas from it.
C preprocessor
The typical C compiler's first pass, which processes
lines beginning with "#" for conditional compilation and
macro definition and does various manipulations of the
program text based on the current definitions. Also
known as cpp(1).
call by reference
An "argument"-passing mechanism in which the "formal
arguments" refer directly to the "actual arguments", and
the "subroutine" can change the actual arguments by
changing the formal arguments. That is, the formal
argument is an "alias" for the actual argument. See
also "call by value".
call by value
An "argument"-passing mechanism in which the "formal
arguments" refer to a copy of the "actual arguments",
and the "subroutine" cannot change the actual arguments
by changing the formal arguments. See also "call by
reference".
callback
A "handler" that you register with some other part of
your program in the hope that the other part of your
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program will "trigger" your handler when some event of
interest transpires.
canonical
Reduced to a standard form to facilitate comparison.
capturing
The use of parentheses around a "subpattern" in a "regu-
lar expression" to store the matched "substring" as a
"backreference". (Captured strings are also returned as
a list in "list context".)
character
A small integer representative of a unit of orthography.
Historically, characters were usually stored as fixed-
width integers (typically in a byte, or maybe two,
depending on the character set), but with the advent of
UTF-8, characters are often stored in a variable number
of bytes depending on the size of the integer that
represents the character. Perl manages this tran-
sparently for you, for the most part.
character class
A square-bracketed list of characters used in a "regular
expression" to indicate that any character of the set
may occur at a given point. Loosely, any predefined set
of characters so used.
character property
A predefined "character class" matchable by the "\p"
"metasymbol". Many standard properties are defined for
"Unicode".
circumfix operator
An "operator" that surrounds its "operand", like the
angle operator, or parentheses, or a hug.
class
A user-defined "type", implemented in Perl via a "pack-
age" that provides (either directly or by inheritance)
methods (that is, subroutines) to handle instances of
the class (its objects). See also "inheritance".
class method
A "method" whose "invocant" is a "package" name, not an
"object" reference. A method associated with the class
as a whole.
client
In networking, a "process" that initiates contact with a
"server" process in order to exchange data and perhaps
receive a service.
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cloister
A "cluster" used to restrict the scope of a "regular
expression modifier".
closure
An "anonymous" subroutine that, when a reference to it
is generated at run time, keeps track of the identities
of externally visible lexical variables even after those
lexical variables have supposedly gone out of "scope".
They're called "closures" because this sort of behavior
gives mathematicians a sense of closure.
cluster
A parenthesized "subpattern" used to group parts of a
"regular expression" into a single "atom".
CODE
The word returned by the ref function when you apply it
to a reference to a subroutine. See also "CV".
code generator
A system that writes code for you in a low-level
language, such as code to implement the backend of a
compiler. See "program generator".
code subpattern
A "regular expression" subpattern whose real purpose is
to execute some Perl code, for example, the "(?{...})"
and "(??{...})" subpatterns.
collating sequence
The order into which characters sort. This is used by
"string" comparison routines to decide, for example,
where in this glossary to put "collating sequence".
command
In "shell" programming, the syntactic combination of a
program name and its arguments. More loosely, anything
you type to a shell (a command interpreter) that starts
it doing something. Even more loosely, a Perl "state-
ment", which might start with a "label" and typically
ends with a semicolon.
command buffering
A mechanism in Perl that lets you store up the output of
each Perl "command" and then flush it out as a single
request to the "operating system". It's enabled by set-
ting the $| ($AUTOFLUSH) variable to a true value. It's
used when you don't want data sitting around not going
where it's supposed to, which may happen because the
default on a "file" or "pipe" is to use "block buffer-
ing".
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command name
The name of the program currently executing, as typed on
the command line. In C, the "command" name is passed to
the program as the first command-line argument. In
Perl, it comes in separately as $0.
command-line arguments
The values you supply along with a program name when you
tell a "shell" to execute a "command". These values are
passed to a Perl program through @ARGV.
comment
A remark that doesn't affect the meaning of the program.
In Perl, a comment is introduced by a "#" character and
continues to the end of the line.
compilation unit
The "file" (or "string", in the case of eval) that is
currently being compiled.
compile phase
Any time before Perl starts running your main program.
See also "run phase". Compile phase is mostly spent in
"compile time", but may also be spent in "run time" when
"BEGIN" blocks, use declarations, or constant subexpres-
sions are being evaluated. The startup and import code
of any use declaration is also run during compile phase.
compile time
The time when Perl is trying to make sense of your code,
as opposed to when it thinks it knows what your code
means and is merely trying to do what it thinks your
code says to do, which is "run time".
compiler
Strictly speaking, a program that munches up another
program and spits out yet another file containing the
program in a "more executable" form, typically contain-
ing native machine instructions. The perl program is
not a compiler by this definition, but it does contain a
kind of compiler that takes a program and turns it into
a more executable form (syntax trees) within the perl
process itself, which the "interpreter" then interprets.
There are, however, extension modules to get Perl to act
more like a "real" compiler. See O.
composer
A "constructor" for a "referent" that isn't really an
"object", like an anonymous array or a hash (or a
sonata, for that matter). For example, a pair of braces
acts as a composer for a hash, and a pair of brackets
acts as a composer for an array. See "Making
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References" in perlref.
concatenation
The process of gluing one cat's nose to another cat's
tail. Also, a similar operation on two strings.
conditional
Something "iffy". See "Boolean context".
connection
In telephony, the temporary electrical circuit between
the caller's and the callee's phone. In networking, the
same kind of temporary circuit between a "client" and a
"server".
construct
As a noun, a piece of syntax made up of smaller pieces.
As a transitive verb, to create an "object" using a
"constructor".
constructor
Any "class method", instance "method", or "subroutine"
that composes, initializes, blesses, and returns an
"object". Sometimes we use the term loosely to mean a
"composer".
context
The surroundings, or environment. The context given by
the surrounding code determines what kind of data a par-
ticular "expression" is expected to return. The three
primary contexts are "list context", "scalar context",
and "void context". Scalar context is sometimes subdi-
vided into "Boolean context", "numeric context", "string
context", and "void context". There's also a "don't
care" scalar context (which is dealt with in Programming
Perl, Third Edition, Chapter 2, "Bits and Pieces" if you
care).
continuation
The treatment of more than one physical "line" as a sin-
gle logical line. "Makefile" lines are continued by
putting a backslash before the "newline". Mail headers
as defined by RFC 822 are continued by putting a space
or tab after the newline. In general, lines in Perl do
not need any form of continuation mark, because "whi-
tespace" (including newlines) is gleefully ignored.
Usually.
core dump
The corpse of a "process", in the form of a file left in
the "working directory" of the process, usually as a
result of certain kinds of fatal error.
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CPAN
The Comprehensive Perl Archive Network. (See "What
modules and extensions are available for Perl? What is
CPAN? What does CPAN/src/... mean?" in perlfaq2).
cracker
Someone who breaks security on computer systems. A
cracker may be a true "hacker" or only a "script kid-
die".
current package
The "package" in which the current statement is com-
piled. Scan backwards in the text of your program
through the current lexical scope or any enclosing lexi-
cal scopes till you find a package declaration. That's
your current package name.
current working directory
See "working directory".
currently selected output channel
The last "filehandle" that was designated with
select("FILEHANDLE"); "STDOUT", if no filehandle has
been selected.
CV An internal "code value" typedef, holding a "subrou-
tine". The "CV" type is a subclass of "SV".
D
dangling statement
A bare, single "statement", without any braces, hanging
off an "if" or "while" conditional. C allows them.
Perl doesn't.
data structure
How your various pieces of data relate to each other and
what shape they make when you put them all together, as
in a rectangular table or a triangular-shaped tree.
data type
A set of possible values, together with all the opera-
tions that know how to deal with those values. For
example, a numeric data type has a certain set of
numbers that you can work with and various mathematical
operations that you can do on the numbers but would make
little sense on, say, a string such as "Kilroy".
Strings have their own operations, such as "concatena-
tion". Compound types made of a number of smaller
pieces generally have operations to compose and decom-
pose them, and perhaps to rearrange them. Objects that
model things in the real world often have operations
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that correspond to real activities. For instance, if
you model an elevator, your elevator object might have
an "open_door()" "method".
datagram
A packet of data, such as a "UDP" message, that (from
the viewpoint of the programs involved) can be sent
independently over the network. (In fact, all packets
are sent independently at the "IP" level, but "stream"
protocols such as "TCP" hide this from your program.)
DBM Stands for "Data Base Management" routines, a set of
routines that emulate an "associative array" using disk
files. The routines use a dynamic hashing scheme to
locate any entry with only two disk accesses. DBM files
allow a Perl program to keep a persistent "hash" across
multiple invocations. You can tie your hash variables
to various DBM implementations--see AnyDBM_File and
DB_File.
declaration
An "assertion" that states something exists and perhaps
describes what it's like, without giving any commitment
as to how or where you'll use it. A declaration is like
the part of your recipe that says, "two cups flour, one
large egg, four or five tadpoles..." See "statement"
for its opposite. Note that some declarations also
function as statements. Subroutine declarations also
act as definitions if a body is supplied.
decrement
To subtract a value from a variable, as in "decrement
$x" (meaning to remove 1 from its value) or "decrement
$x by 3".
default
A "value" chosen for you if you don't supply a value of
your own.
defined
Having a meaning. Perl thinks that some of the things
people try to do are devoid of meaning, in particular,
making use of variables that have never been given a
"value" and performing certain operations on data that
isn't there. For example, if you try to read data past
the end of a file, Perl will hand you back an undefined
value. See also "false" and "defined" in perlfunc.
delimiter
A "character" or "string" that sets bounds to an
arbitrarily-sized textual object, not to be confused
with a "separator" or "terminator". "To delimit" really
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just means "to surround" or "to enclose" (like these
parentheses are doing).
dereference
A fancy computer science term meaning "to follow a
"reference" to what it points to". The "de" part of it
refers to the fact that you're taking away one level of
"indirection".
derived class
A "class" that defines some of its methods in terms of a
more generic class, called a "base class". Note that
classes aren't classified exclusively into base classes
or derived classes: a class can function as both a
derived class and a base class simultaneously, which is
kind of classy.
descriptor
See "file descriptor".
destroy
To deallocate the memory of a "referent" (first trigger-
ing its "DESTROY" method, if it has one).
destructor
A special "method" that is called when an "object" is
thinking about destroying itself. A Perl program's
"DESTROY" method doesn't do the actual destruction; Perl
just triggers the method in case the "class" wants to do
any associated cleanup.
device
A whiz-bang hardware gizmo (like a disk or tape drive or
a modem or a joystick or a mouse) attached to your com-
puter, that the "operating system" tries to make look
like a "file" (or a bunch of files). Under Unix, these
fake files tend to live in the /dev directory.
directive
A "pod" directive. See perlpod.
directory
A special file that contains other files. Some operat-
ing systems call these "folders", "drawers", or "cata-
logs".
directory handle
A name that represents a particular instance of opening
a directory to read it, until you close it. See the
opendir function.
dispatch
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To send something to its correct destination. Often
used metaphorically to indicate a transfer of program-
matic control to a destination selected algorithmically,
often by lookup in a table of function references or, in
the case of object methods, by traversing the inheri-
tance tree looking for the most specific definition for
the method.
distribution
A standard, bundled release of a system of software.
The default usage implies source code is included. If
that is not the case, it will be called a "binary-only"
distribution.
dweomer
An enchantment, illusion, phantasm, or jugglery. Said
when Perl's magical "dwimmer" effects don't do what you
expect, but rather seem to be the product of arcane
dweomercraft, sorcery, or wonder working. [From Old
English]
dwimmer
DWIM is an acronym for "Do What I Mean", the principle
that something should just do what you want it to do
without an undue amount of fuss. A bit of code that does
"dwimming" is a "dwimmer". Dwimming can require a great
deal of behind-the-scenes magic, which (if it doesn't
stay properly behind the scenes) is called a "dweomer"
instead.
dynamic scoping
Dynamic scoping works over a dynamic scope, making vari-
ables visible throughout the rest of the "block" in
which they are first used and in any subroutines that
are called by the rest of the block. Dynamically scoped
variables can have their values temporarily changed (and
implicitly restored later) by a local operator. (Com-
pare "lexical scoping".) Used more loosely to mean how
a subroutine that is in the middle of calling another
subroutine "contains" that subroutine at "run time".
E
eclectic
Derived from many sources. Some would say too many.
element
A basic building block. When you're talking about an
"array", it's one of the items that make up the array.
embedding
When something is contained in something else,
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particularly when that might be considered surprising:
"I've embedded a complete Perl interpreter in my edi-
tor!"
empty subclass test
The notion that an empty "derived class" should behave
exactly like its "base class".
en passant
When you change a "value" as it is being copied. [From
French, "in passing", as in the exotic pawn-capturing
maneuver in chess.]
encapsulation
The veil of abstraction separating the "interface" from
the "implementation" (whether enforced or not), which
mandates that all access to an "object"'s state be
through methods alone.
endian
See "little-endian" and "big-endian".
environment
The collective set of environment variables your "pro-
cess" inherits from its parent. Accessed via %ENV.
environment variable
A mechanism by which some high-level agent such as a
user can pass its preferences down to its future offspr-
ing (child processes, grandchild processes, great-
grandchild processes, and so on). Each environment
variable is a "key"/"value" pair, like one entry in a
"hash".
EOF End of File. Sometimes used metaphorically as the ter-
minating string of a "here document".
errno
The error number returned by a "syscall" when it fails.
Perl refers to the error by the name $! (or $OS_ERROR if
you use the English module).
error
See "exception" or "fatal error".
escape sequence
See "metasymbol".
exception
A fancy term for an error. See "fatal error".
exception handling
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The way a program responds to an error. The exception
handling mechanism in Perl is the eval operator.
exec
To throw away the current "process"'s program and
replace it with another without exiting the process or
relinquishing any resources held (apart from the old
memory image).
executable file
A "file" that is specially marked to tell the "operating
system" that it's okay to run this file as a program.
Usually shortened to "executable".
execute
To run a program or "subroutine". (Has nothing to do
with the kill built-in, unless you're trying to run a
"signal handler".)
execute bit
The special mark that tells the operating system it can
run this program. There are actually three execute bits
under Unix, and which bit gets used depends on whether
you own the file singularly, collectively, or not at
all.
exit status
See "status".
export
To make symbols from a "module" available for "import"
by other modules.
expression
Anything you can legally say in a spot where a "value"
is required. Typically composed of literals, variables,
operators, functions, and "subroutine" calls, not neces-
sarily in that order.
extension
A Perl module that also pulls in compiled C or C++ code.
More generally, any experimental option that can be com-
piled into Perl, such as multithreading.
F
false
In Perl, any value that would look like "" or "0" if
evaluated in a string context. Since undefined values
evaluate to "", all undefined values are false, but not
all false values are undefined.
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FAQ Frequently Asked Question (although not necessarily fre-
quently answered, especially if the answer appears in
the Perl FAQ shipped standard with Perl).
fatal error
An uncaught "exception", which causes termination of the
"process" after printing a message on your "standard
error" stream. Errors that happen inside an eval are
not fatal. Instead, the eval terminates after placing
the exception message in the $@ ($EVAL_ERROR) variable.
You can try to provoke a fatal error with the die opera-
tor (known as throwing or raising an exception), but
this may be caught by a dynamically enclosing eval. If
not caught, the die becomes a fatal error.
field
A single piece of numeric or string data that is part of
a longer "string", "record", or "line". Variable-width
fields are usually split up by separators (so use split
to extract the fields), while fixed-width fields are
usually at fixed positions (so use unpack). Instance
variables are also known as fields.
FIFO
First In, First Out. See also "LIFO". Also, a nickname
for a "named pipe".
file
A named collection of data, usually stored on disk in a
"directory" in a "filesystem". Roughly like a document,
if you're into office metaphors. In modern filesystems,
you can actually give a file more than one name. Some
files have special properties, like directories and dev-
ices.
file descriptor
The little number the "operating system" uses to keep
track of which opened "file" you're talking about. Perl
hides the file descriptor inside a "standard I/O" stream
and then attaches the stream to a "filehandle".
file test operator
A built-in unary operator that you use to determine
whether something is "true" about a file, such as "-o
$filename" to test whether you're the owner of the file.
fileglob
A "wildcard" match on filenames. See the glob function.
filehandle
An identifier (not necessarily related to the real name
of a file) that represents a particular instance of
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opening a file until you close it. If you're going to
open and close several different files in succession,
it's fine to open each of them with the same filehandle,
so you don't have to write out separate code to process
each file.
filename
One name for a file. This name is listed in a "direc-
tory", and you can use it in an open to tell the
"operating system" exactly which file you want to open,
and associate the file with a "filehandle" which will
carry the subsequent identity of that file in your pro-
gram, until you close it.
filesystem
A set of directories and files residing on a partition
of the disk. Sometimes known as a "partition". You can
change the file's name or even move a file around from
directory to directory within a filesystem without actu-
ally moving the file itself, at least under Unix.
filter
A program designed to take a "stream" of input and
transform it into a stream of output.
flag
We tend to avoid this term because it means so many
things. It may mean a command-line "switch" that takes
no argument itself (such as Perl's -n and -p flags) or,
less frequently, a single-bit indicator (such as the
"O_CREAT" and "O_EXCL" flags used in sysopen).
floating point
A method of storing numbers in "scientific notation",
such that the precision of the number is independent of
its magnitude (the decimal point "floats"). Perl does
its numeric work with floating-point numbers (sometimes
called "floats"), when it can't get away with using
integers. Floating-point numbers are mere approxima-
tions of real numbers.
flush
The act of emptying a "buffer", often before it's full.
FMTEYEWTK
Far More Than Everything You Ever Wanted To Know. An
exhaustive treatise on one narrow topic, something of a
super-"FAQ". See Tom for far more.
fork
To create a child "process" identical to the parent pro-
cess at its moment of conception, at least until it gets
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ideas of its own. A thread with protected memory.
formal arguments
The generic names by which a "subroutine" knows its
arguments. In many languages, formal arguments are
always given individual names, but in Perl, the formal
arguments are just the elements of an array. The formal
arguments to a Perl program are $ARGV[0], $ARGV[1], and
so on. Similarly, the formal arguments to a Perl sub-
routine are $_[0], $_[1], and so on. You may give the
arguments individual names by assigning the values to a
my list. See also "actual arguments".
format
A specification of how many spaces and digits and things
to put somewhere so that whatever you're printing comes
out nice and pretty.
freely available
Means you don't have to pay money to get it, but the
copyright on it may still belong to someone else (like
Larry).
freely redistributable
Means you're not in legal trouble if you give a bootleg
copy of it to your friends and we find out about it. In
fact, we'd rather you gave a copy to all your friends.
freeware
Historically, any software that you give away, particu-
larly if you make the source code available as well.
Now often called "open source software". Recently there
has been a trend to use the term in contradistinction to
"open source software", to refer only to free software
released under the Free Software Foundation's GPL (Gen-
eral Public License), but this is difficult to justify
etymologically.
function
Mathematically, a mapping of each of a set of input
values to a particular output value. In computers,
refers to a "subroutine" or "operator" that returns a
"value". It may or may not have input values (called
arguments).
funny character
Someone like Larry, or one of his peculiar friends.
Also refers to the strange prefixes that Perl requires
as noun markers on its variables.
garbage collection
A misnamed feature--it should be called, "expecting your
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mother to pick up after you". Strictly speaking, Perl
doesn't do this, but it relies on a reference-counting
mechanism to keep things tidy. However, we rarely speak
strictly and will often refer to the reference-counting
scheme as a form of garbage collection. (If it's any
comfort, when your interpreter exits, a "real" garbage
collector runs to make sure everything is cleaned up if
you've been messy with circular references and such.)
G
GID Group ID--in Unix, the numeric group ID that the
"operating system" uses to identify you and members of
your "group".
glob
Strictly, the shell's "*" character, which will match a
"glob" of characters when you're trying to generate a
list of filenames. Loosely, the act of using globs and
similar symbols to do pattern matching. See also
"fileglob" and "typeglob".
global
Something you can see from anywhere, usually used of
variables and subroutines that are visible everywhere in
your program. In Perl, only certain special variables
are truly global--most variables (and all subroutines)
exist only in the current "package". Global variables
can be declared with our. See "our" in perlfunc.
global destruction
The "garbage collection" of globals (and the running of
any associated object destructors) that takes place when
a Perl "interpreter" is being shut down. Global des-
truction should not be confused with the Apocalypse,
except perhaps when it should.
glue language
A language such as Perl that is good at hooking things
together that weren't intended to be hooked together.
granularity
The size of the pieces you're dealing with, mentally
speaking.
greedy
A "subpattern" whose "quantifier" wants to match as many
things as possible.
grep
Originally from the old Unix editor command for "Glo-
bally search for a Regular Expression and Print it", now
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used in the general sense of any kind of search, espe-
cially text searches. Perl has a built-in grep function
that searches a list for elements matching any given
criterion, whereas the grep(1) program searches for
lines matching a "regular expression" in one or more
files.
group
A set of users of which you are a member. In some
operating systems (like Unix), you can give certain file
access permissions to other members of your group.
GV An internal "glob value" typedef, holding a "typeglob".
The "GV" type is a subclass of "SV".
H
hacker
Someone who is brilliantly persistent in solving techni-
cal problems, whether these involve golfing, fighting
orcs, or programming. Hacker is a neutral term, morally
speaking. Good hackers are not to be confused with evil
crackers or clueless script kiddies. If you confuse
them, we will presume that you are either evil or clue-
less.
handler
A "subroutine" or "method" that is called by Perl when
your program needs to respond to some internal event,
such as a "signal", or an encounter with an operator
subject to "operator overloading". See also "callback".
hard reference
A "scalar" "value" containing the actual address of a
"referent", such that the referent's "reference" count
accounts for it. (Some hard references are held inter-
nally, such as the implicit reference from one of a
"typeglob"'s variable slots to its corresponding
referent.) A hard reference is different from a "sym-
bolic reference".
hash
An unordered association of "key"/"value" pairs, stored
such that you can easily use a string "key" to look up
its associated data "value". This glossary is like a
hash, where the word to be defined is the key, and the
definition is the value. A hash is also sometimes sep-
tisyllabically called an "associative array", which is a
pretty good reason for simply calling it a "hash"
instead.
hash table
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A data structure used internally by Perl for implement-
ing associative arrays (hashes) efficiently. See also
"bucket".
header file
A file containing certain required definitions that you
must include "ahead" of the rest of your program to do
certain obscure operations. A C header file has a .h
extension. Perl doesn't really have header files,
though historically Perl has sometimes used translated
.h files with a .ph extension. See "require" in perl-
func. (Header files have been superseded by the "module"
mechanism.)
here document
So called because of a similar construct in shells that
pretends that the lines following the "command" are a
separate "file" to be fed to the command, up to some
terminating string. In Perl, however, it's just a fancy
form of quoting.
hexadecimal
A number in base 16, "hex" for short. The digits for 10
through 16 are customarily represented by the letters
"a" through "f". Hexadecimal constants in Perl start
with "0x". See also "hex" in perlfunc.
home directory
The directory you are put into when you log in. On a
Unix system, the name is often placed into $ENV{HOME} or
$ENV{LOGDIR} by login, but you can also find it with
"(getpwuid($<))[7]". (Some platforms do not have a con-
cept of a home directory.)
host
The computer on which a program or other data resides.
hubris
Excessive pride, the sort of thing Zeus zaps you for.
Also the quality that makes you write (and maintain)
programs that other people won't want to say bad things
about. Hence, the third great virtue of a programmer.
See also "laziness" and "impatience".
HV Short for a "hash value" typedef, which holds Perl's
internal representation of a hash. The "HV" type is a
subclass of "SV".
I
identifier
A legally formed name for most anything in which a
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computer program might be interested. Many languages
(including Perl) allow identifiers that start with a
letter and contain letters and digits. Perl also counts
the underscore character as a valid letter. (Perl also
has more complicated names, such as "qualified" names.)
impatience
The anger you feel when the computer is being lazy.
This makes you write programs that don't just react to
your needs, but actually anticipate them. Or at least
that pretend to. Hence, the second great virtue of a
programmer. See also "laziness" and "hubris".
implementation
How a piece of code actually goes about doing its job.
Users of the code should not count on implementation
details staying the same unless they are part of the
published "interface".
import
To gain access to symbols that are exported from another
module. See "use" in perlfunc.
increment
To increase the value of something by 1 (or by some
other number, if so specified).
indexing
In olden days, the act of looking up a "key" in an
actual index (such as a phone book), but now merely the
act of using any kind of key or position to find the
corresponding "value", even if no index is involved.
Things have degenerated to the point that Perl's index
function merely locates the position (index) of one
string in another.
indirect filehandle
An "expression" that evaluates to something that can be
used as a "filehandle": a "string" (filehandle name), a
"typeglob", a typeglob "reference", or a low-level "IO"
object.
indirect object
In English grammar, a short noun phrase between a verb
and its direct object indicating the beneficiary or
recipient of the action. In Perl, "print STDOUT
"$foo\n";" can be understood as "verb indirect-object
object" where "STDOUT" is the recipient of the print
action, and "$foo" is the object being printed. Simi-
larly, when invoking a "method", you might place the
invocant between the method and its arguments:
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$gollum = new Pathetic::Creature "Smeagol";
give $gollum "Fisssssh!";
give $gollum "Precious!";
indirect object slot
The syntactic position falling between a method call and
its arguments when using the indirect object invocation
syntax. (The slot is distinguished by the absence of a
comma between it and the next argument.) "STDERR" is in
the indirect object slot here:
print STDERR "Awake! Awake! Fear, Fire,
Foes! Awake!\n";
indirection
If something in a program isn't the value you're looking
for but indicates where the value is, that's indirec-
tion. This can be done with either symbolic references
or hard references.
infix
An "operator" that comes in between its operands, such
as multiplication in "24 * 7".
inheritance
What you get from your ancestors, genetically or other-
wise. If you happen to be a "class", your ancestors are
called base classes and your descendants are called
derived classes. See "single inheritance" and "multiple
inheritance".
instance
Short for "an instance of a class", meaning an "object"
of that "class".
instance variable
An "attribute" of an "object"; data stored with the par-
ticular object rather than with the class as a whole.
integer
A number with no fractional (decimal) part. A counting
number, like 1, 2, 3, and so on, but including 0 and the
negatives.
interface
The services a piece of code promises to provide for-
ever, in contrast to its "implementation", which it
should feel free to change whenever it likes.
interpolation
The insertion of a scalar or list value somewhere in the
middle of another value, such that it appears to have
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been there all along. In Perl, variable interpolation
happens in double-quoted strings and patterns, and list
interpolation occurs when constructing the list of
values to pass to a list operator or other such con-
struct that takes a "LIST".
interpreter
Strictly speaking, a program that reads a second program
and does what the second program says directly without
turning the program into a different form first, which
is what compilers do. Perl is not an interpreter by
this definition, because it contains a kind of compiler
that takes a program and turns it into a more executable
form (syntax trees) within the perl process itself,
which the Perl "run time" system then interprets.
invocant
The agent on whose behalf a "method" is invoked. In a
"class" method, the invocant is a package name. In an
"instance" method, the invocant is an object reference.
invocation
The act of calling up a deity, daemon, program, method,
subroutine, or function to get it do what you think it's
supposed to do. We usually "call" subroutines but
"invoke" methods, since it sounds cooler.
I/O Input from, or output to, a "file" or "device".
IO An internal I/O object. Can also mean "indirect
object".
IP Internet Protocol, or Intellectual Property.
IPC Interprocess Communication.
is-a
A relationship between two objects in which one object
is considered to be a more specific version of the
other, generic object: "A camel is a mammal." Since the
generic object really only exists in a Platonic sense,
we usually add a little abstraction to the notion of
objects and think of the relationship as being between a
generic "base class" and a specific "derived class".
Oddly enough, Platonic classes don't always have Pla-
tonic relationships--see "inheritance".
iteration
Doing something repeatedly.
iterator
A special programming gizmo that keeps track of where
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you are in something that you're trying to iterate over.
The "foreach" loop in Perl contains an iterator; so does
a hash, allowing you to each through it.
IV The integer four, not to be confused with six, Tom's
favorite editor. IV also means an internal Integer Value
of the type a "scalar" can hold, not to be confused with
an "NV".
J
JAPH
"Just Another Perl Hacker," a clever but cryptic bit of
Perl code that when executed, evaluates to that string.
Often used to illustrate a particular Perl feature, and
something of an ungoing Obfuscated Perl Contest seen in
Usenix signatures.
K
key The string index to a "hash", used to look up the
"value" associated with that key.
keyword
See "reserved words".
L
label
A name you give to a "statement" so that you can talk
about that statement elsewhere in the program.
laziness
The quality that makes you go to great effort to reduce
overall energy expenditure. It makes you write labor-
saving programs that other people will find useful, and
document what you wrote so you don't have to answer so
many questions about it. Hence, the first great virtue
of a programmer. Also hence, this book. See also
"impatience" and "hubris".
left shift
A "bit shift" that multiplies the number by some power
of 2.
leftmost longest
The preference of the "regular expression" engine to
match the leftmost occurrence of a "pattern", then given
a position at which a match will occur, the preference
for the longest match (presuming the use of a "greedy"
quantifier). See perlre for much more on this subject.
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lexeme
Fancy term for a "token".
lexer
Fancy term for a "tokener".
lexical analysis
Fancy term for "tokenizing".
lexical scoping
Looking at your Oxford English Dictionary through a
microscope. (Also known as "static scoping", because
dictionaries don't change very fast.) Similarly, look-
ing at variables stored in a private dictionary
(namespace) for each scope, which are visible only from
their point of declaration down to the end of the lexi-
cal scope in which they are declared. --Syn. "static
scoping". --Ant. "dynamic scoping".
lexical variable
A "variable" subject to "lexical scoping", declared by
my. Often just called a "lexical". (The our declara-
tion declares a lexically scoped name for a global vari-
able, which is not itself a lexical variable.)
library
Generally, a collection of procedures. In ancient days,
referred to a collection of subroutines in a .pl file.
In modern times, refers more often to the entire collec-
tion of Perl modules on your system.
LIFO
Last In, First Out. See also "FIFO". A LIFO is usually
called a "stack".
line
In Unix, a sequence of zero or more non-newline charac-
ters terminated with a "newline" character. On non-Unix
machines, this is emulated by the C library even if the
underlying "operating system" has different ideas.
line buffering
Used by a "standard I/O" output stream that flushes its
"buffer" after every "newline". Many standard I/O
libraries automatically set up line buffering on output
that is going to the terminal.
line number
The number of lines read previous to this one, plus 1.
Perl keeps a separate line number for each source or
input file it opens. The current source file's line
number is represented by "__LINE__". The current input
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line number (for the file that was most recently read
via "<FH>") is represented by the $.
($INPUT_LINE_NUMBER) variable. Many error messages
report both values, if available.
link
Used as a noun, a name in a "directory", representing a
"file". A given file can have multiple links to it.
It's like having the same phone number listed in the
phone directory under different names. As a verb, to
resolve a partially compiled file's unresolved symbols
into a (nearly) executable image. Linking can generally
be static or dynamic, which has nothing to do with
static or dynamic scoping.
LIST
A syntactic construct representing a comma-separated
list of expressions, evaluated to produce a "list
value". Each "expression" in a "LIST" is evaluated in
"list context" and interpolated into the list value.
list
An ordered set of scalar values.
list context
The situation in which an "expression" is expected by
its surroundings (the code calling it) to return a list
of values rather than a single value. Functions that
want a "LIST" of arguments tell those arguments that
they should produce a list value. See also "context".
list operator
An "operator" that does something with a list of values,
such as join or grep. Usually used for named built-in
operators (such as print, unlink, and system) that do
not require parentheses around their "argument" list.
list value
An unnamed list of temporary scalar values that may be
passed around within a program from any list-generating
function to any function or construct that provides a
"list context".
literal
A token in a programming language such as a number or
"string" that gives you an actual "value" instead of
merely representing possible values as a "variable"
does.
little-endian
From Swift: someone who eats eggs little end first.
Also used of computers that store the least significant
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"byte" of a word at a lower byte address than the most
significant byte. Often considered superior to big-
endian machines. See also "big-endian".
local
Not meaning the same thing everywhere. A global vari-
able in Perl can be localized inside a dynamic scope via
the local operator.
logical operator
Symbols representing the concepts "and", "or", "xor",
and "not".
lookahead
An "assertion" that peeks at the string to the right of
the current match location.
lookbehind
An "assertion" that peeks at the string to the left of
the current match location.
loop
A construct that performs something repeatedly, like a
roller coaster.
loop control statement
Any statement within the body of a loop that can make a
loop prematurely stop looping or skip an "iteration".
Generally you shouldn't try this on roller coasters.
loop label
A kind of key or name attached to a loop (or roller
coaster) so that loop control statements can talk about
which loop they want to control.
lvaluable
Able to serve as an "lvalue".
lvalue
Term used by language lawyers for a storage location you
can assign a new "value" to, such as a "variable" or an
element of an "array". The "l" is short for "left", as
in the left side of an assignment, a typical place for
lvalues. An "lvaluable" function or expression is one
to which a value may be assigned, as in "pos($x) = 10".
lvalue modifier
An adjectival pseudofunction that warps the meaning of
an "lvalue" in some declarative fashion. Currently
there are three lvalue modifiers: my, our, and local.
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M
magic
Technically speaking, any extra semantics attached to a
variable such as $!, $0, %ENV, or %SIG, or to any tied
variable. Magical things happen when you diddle those
variables.
magical increment
An "increment" operator that knows how to bump up alpha-
betics as well as numbers.
magical variables
Special variables that have side effects when you access
them or assign to them. For example, in Perl, changing
elements of the %ENV array also changes the correspond-
ing environment variables that subprocesses will use.
Reading the $! variable gives you the current system
error number or message.
Makefile
A file that controls the compilation of a program. Perl
programs don't usually need a "Makefile" because the
Perl compiler has plenty of self-control.
man The Unix program that displays online documentation
(manual pages) for you.
manpage
A "page" from the manuals, typically accessed via the
man(1) command. A manpage contains a SYNOPSIS, a
DESCRIPTION, a list of BUGS, and so on, and is typically
longer than a page. There are manpages documenting com-
mands, syscalls, "library" functions, devices, proto-
cols, files, and such. In this book, we call any piece
of standard Perl documentation (like perlop or
perldelta) a manpage, no matter what format it's
installed in on your system.
matching
See "pattern matching".
member data
See "instance variable".
memory
This always means your main memory, not your disk.
Clouding the issue is the fact that your machine may
implement "virtual" memory; that is, it will pretend
that it has more memory than it really does, and it'll
use disk space to hold inactive bits. This can make it
seem like you have a little more memory than you really
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do, but it's not a substitute for real memory. The best
thing that can be said about virtual memory is that it
lets your performance degrade gradually rather than sud-
denly when you run out of real memory. But your program
can die when you run out of virtual memory too, if you
haven't thrashed your disk to death first.
metacharacter
A "character" that is not supposed to be treated nor-
mally. Which characters are to be treated specially as
metacharacters varies greatly from context to context.
Your "shell" will have certain metacharacters, double-
quoted Perl strings have other metacharacters, and "reg-
ular expression" patterns have all the double-quote
metacharacters plus some extra ones of their own.
metasymbol
Something we'd call a "metacharacter" except that it's a
sequence of more than one character. Generally, the
first character in the sequence must be a true metachar-
acter to get the other characters in the metasymbol to
misbehave along with it.
method
A kind of action that an "object" can take if you tell
it to. See perlobj.
minimalism
The belief that "small is beautiful." Paradoxically, if
you say something in a small language, it turns out big,
and if you say it in a big language, it turns out small.
Go figure.
mode
In the context of the stat syscall, refers to the field
holding the "permission bits" and the type of the
"file".
modifier
See "statement modifier", "regular expression modifier",
and "lvalue modifier", not necessarily in that order.
module
A "file" that defines a "package" of (almost) the same
name, which can either "export" symbols or function as
an "object" class. (A module's main .pm file may also
load in other files in support of the module.) See the
use built-in.
modulus
An integer divisor when you're interested in the
remainder instead of the quotient.
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monger
Short for Perl Monger, a purveyor of Perl.
mortal
A temporary value scheduled to die when the current
statement finishes.
multidimensional array
An array with multiple subscripts for finding a single
element. Perl implements these using references--see
perllol and perldsc.
multiple inheritance
The features you got from your mother and father, mixed
together unpredictably. (See also "inheritance", and
"single inheritance".) In computer languages (including
Perl), the notion that a given class may have multiple
direct ancestors or base classes.
N
named pipe
A "pipe" with a name embedded in the "filesystem" so
that it can be accessed by two unrelated processes.
namespace
A domain of names. You needn't worry about whether the
names in one such domain have been used in another. See
"package".
network address
The most important attribute of a socket, like your
telephone's telephone number. Typically an IP address.
See also "port".
newline
A single character that represents the end of a line,
with the ASCII value of 012 octal under Unix (but 015 on
a Mac), and represented by "\n" in Perl strings. For
Windows machines writing text files, and for certain
physical devices like terminals, the single newline gets
automatically translated by your C library into a line
feed and a carriage return, but normally, no translation
is done.
NFS Network Filesystem, which allows you to mount a remote
filesystem as if it were local.
null character
A character with the ASCII value of zero. It's used by
C to terminate strings, but Perl allows strings to con-
tain a null.
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null list
A "list value" with zero elements, represented in Perl
by "()".
null string
A "string" containing no characters, not to be confused
with a string containing a "null character", which has a
positive length and is "true".
numeric context
The situation in which an expression is expected by its
surroundings (the code calling it) to return a number.
See also "context" and "string context".
NV Short for Nevada, no part of which will ever be confused
with civilization. NV also means an internal floating-
point Numeric Value of the type a "scalar" can hold, not
to be confused with an "IV".
nybble
Half a "byte", equivalent to one "hexadecimal" digit,
and worth four bits.
O
object
An "instance" of a "class". Something that "knows" what
user-defined type (class) it is, and what it can do
because of what class it is. Your program can request
an object to do things, but the object gets to decide
whether it wants to do them or not. Some objects are
more accommodating than others.
octal
A number in base 8. Only the digits 0 through 7 are
allowed. Octal constants in Perl start with 0, as in
013. See also the oct function.
offset
How many things you have to skip over when moving from
the beginning of a string or array to a specific posi-
tion within it. Thus, the minimum offset is zero, not
one, because you don't skip anything to get to the first
item.
one-liner
An entire computer program crammed into one line of
text.
open source software
Programs for which the source code is freely available
and freely redistributable, with no commercial strings
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attached. For a more detailed definition, see
<http://www.opensource.org/osd.html>.
operand
An "expression" that yields a "value" that an "operator"
operates on. See also "precedence".
operating system
A special program that runs on the bare machine and
hides the gory details of managing processes and dev-
ices. Usually used in a looser sense to indicate a par-
ticular culture of programming. The loose sense can be
used at varying levels of specificity. At one extreme,
you might say that all versions of Unix and Unix-
lookalikes are the same operating system (upsetting many
people, especially lawyers and other advocates). At the
other extreme, you could say this particular version of
this particular vendor's operating system is different
from any other version of this or any other vendor's
operating system. Perl is much more portable across
operating systems than many other languages. See also
"architecture" and "platform".
operator
A gizmo that transforms some number of input values to
some number of output values, often built into a
language with a special syntax or symbol. A given
operator may have specific expectations about what types
of data you give as its arguments (operands) and what
type of data you want back from it.
operator overloading
A kind of "overloading" that you can do on built-in
operators to make them work on objects as if the objects
were ordinary scalar values, but with the actual seman-
tics supplied by the object class. This is set up with
the overload "pragma".
options
See either switches or "regular expression modifier".
overloading
Giving additional meanings to a symbol or construct.
Actually, all languages do overloading to one extent or
another, since people are good at figuring out things
from "context".
overriding
Hiding or invalidating some other definition of the same
name. (Not to be confused with "overloading", which
adds definitions that must be disambiguated some other
way.) To confuse the issue further, we use the word with
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two overloaded definitions: to describe how you can
define your own "subroutine" to hide a built-in "func-
tion" of the same name (see "Overriding Built-in Func-
tions" in perlsub) and to describe how you can define a
replacement "method" in a "derived class" to hide a
"base class"'s method of the same name (see perlobj).
owner
The one user (apart from the superuser) who has absolute
control over a "file". A file may also have a "group"
of users who may exercise joint ownership if the real
owner permits it. See "permission bits".
P
package
A "namespace" for global variables, subroutines, and the
like, such that they can be kept separate from like-
named symbols in other namespaces. In a sense, only the
package is global, since the symbols in the package's
symbol table are only accessible from code compiled out-
side the package by naming the package. But in another
sense, all package symbols are also globals--they're
just well-organized globals.
pad Short for "scratchpad".
parameter
See "argument".
parent class
See "base class".
parse tree
See "syntax tree".
parsing
The subtle but sometimes brutal art of attempting to
turn your possibly malformed program into a valid "syn-
tax tree".
patch
To fix by applying one, as it were. In the realm of
hackerdom, a listing of the differences between two ver-
sions of a program as might be applied by the patch(1)
program when you want to fix a bug or upgrade your old
version.
PATH
The list of directories the system searches to find a
program you want to "execute". The list is stored as
one of your environment variables, accessible in Perl as
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$ENV{PATH}.
pathname
A fully qualified filename such as /usr/bin/perl. Some-
times confused with "PATH".
pattern
A template used in "pattern matching".
pattern matching
Taking a pattern, usually a "regular expression", and
trying the pattern various ways on a string to see
whether there's any way to make it fit. Often used to
pick interesting tidbits out of a file.
permission bits
Bits that the "owner" of a file sets or unsets to allow
or disallow access to other people. These flag bits are
part of the "mode" word returned by the stat built-in
when you ask about a file. On Unix systems, you can
check the ls(1) manpage for more information.
Pern
What you get when you do "Perl++" twice. Doing it only
once will curl your hair. You have to increment it
eight times to shampoo your hair. Lather, rinse,
iterate.
pipe
A direct "connection" that carries the output of one
"process" to the input of another without an intermedi-
ate temporary file. Once the pipe is set up, the two
processes in question can read and write as if they were
talking to a normal file, with some caveats.
pipeline
A series of processes all in a row, linked by pipes,
where each passes its output stream to the next.
platform
The entire hardware and software context in which a pro-
gram runs. A
program written in a platform-dependent language might
break if you change any of: machine, operating system,
libraries, compiler, or system configuration. The perl
interpreter has to be compiled differently for each
platform because it is implemented in C, but programs
written in the Perl language are largely
platform-independent.
pod The markup used to embed documentation into your Perl
code. See perlpod.
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pointer
A "variable" in a language like C that contains the
exact memory location of some other item. Perl handles
pointers internally so you don't have to worry about
them. Instead, you just use symbolic pointers in the
form of keys and "variable" names, or hard references,
which aren't pointers (but act like pointers and do in
fact contain pointers).
polymorphism
The notion that you can tell an "object" to do something
generic, and the object will interpret the command in
different ways depending on its type. [<Gk many shapes]
port
The part of the address of a TCP or UDP socket that
directs packets to the correct process after finding the
right machine, something like the phone extension you
give when you reach the company operator. Also, the
result of converting code to run on a different platform
than originally intended, or the verb denoting this
conversion.
portable
Once upon a time, C code compilable under both BSD and
SysV. In general, code that can be easily converted to
run on another "platform", where "easily" can be defined
however you like, and usually is. Anything may be con-
sidered portable if you try hard enough. See mobile
home or London Bridge.
porter
Someone who "carries" software from one "platform" to
another. Porting programs written in platform-dependent
languages such as C can be difficult work, but porting
programs like Perl is very much worth the agony.
POSIX
The Portable Operating System Interface specification.
postfix
An "operator" that follows its "operand", as in "$x++".
pp An internal shorthand for a "push-pop" code, that is, C
code implementing Perl's stack machine.
pragma
A standard module whose practical hints and suggestions
are received (and possibly ignored) at compile time.
Pragmas are named in all lowercase.
precedence
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The rules of conduct that, in the absence of other gui-
dance, determine what should happen first. For example,
in the absence of parentheses, you always do multiplica-
tion before addition.
prefix
An "operator" that precedes its "operand", as in "++$x".
preprocessing
What some helper "process" did to transform the incoming
data into a form more suitable for the current process.
Often done with an incoming "pipe". See also "C prepro-
cessor".
procedure
A "subroutine".
process
An instance of a running program. Under multitasking
systems like Unix, two or more separate processes could
be running the same program independently at the same
time--in fact, the fork function is designed to bring
about this happy state of affairs. Under other operating
systems, processes are sometimes called "threads",
"tasks", or "jobs", often with slight nuances in mean-
ing.
program generator
A system that algorithmically writes code for you in a
high-level language. See also "code generator".
progressive matching
Pattern matching that picks up where it left off before.
property
See either "instance variable" or "character property".
protocol
In networking, an agreed-upon way of sending messages
back and forth so that neither correspondent will get
too confused.
prototype
An optional part of a "subroutine" declaration telling
the Perl compiler how many and what flavor of arguments
may be passed as "actual arguments", so that you can
write subroutine calls that parse much like built-in
functions. (Or don't parse, as the case may be.)
pseudofunction
A construct that sometimes looks like a function but
really isn't. Usually reserved for "lvalue" modifiers
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like my, for "context" modifiers like scalar, and for
the pick-your-own-quotes constructs, "q//", "qq//",
"qx//", "qw//", "qr//", "m//", "s///", "y///", and
"tr///".
pseudohash
A reference to an array whose initial element happens to
hold a reference to a hash. You can treat a pseudohash
reference as either an array reference or a hash refer-
ence.
pseudoliteral
An "operator" that looks something like a "literal",
such as the output-grabbing operator, "`""command""`".
public domain
Something not owned by anybody. Perl is copyrighted and
is thus not in the public domain--it's just "freely
available" and "freely redistributable".
pumpkin
A notional "baton" handed around the Perl community
indicating who is the lead integrator in some arena of
development.
pumpking
A "pumpkin" holder, the person in charge of pumping the
pump, or at least priming it. Must be willing to play
the part of the Great Pumpkin now and then.
PV A "pointer value", which is Perl Internals Talk for a
"char*".
Q
qualified
Possessing a complete name. The symbol $Ent::moot is
qualified; $moot is unqualified. A fully qualified
filename is specified from the top-level directory.
quantifier
A component of a "regular expression" specifying how
many times the foregoing "atom" may occur.
R
readable
With respect to files, one that has the proper permis-
sion bit set to let you access the file. With respect
to computer programs, one that's written well enough
that someone has a chance of figuring out what it's try-
ing to do.
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reaping
The last rites performed by a parent "process" on behalf
of a deceased child process so that it doesn't remain a
"zombie". See the wait and waitpid function calls.
record
A set of related data values in a "file" or "stream",
often associated with a unique "key" field. In Unix,
often commensurate with a "line", or a blank-line-
terminated set of lines (a "paragraph"). Each line of
the /etc/passwd file is a record, keyed on login name,
containing information about that user.
recursion
The art of defining something (at least partly) in terms
of itself, which is a naughty no-no in dictionaries but
often works out okay in computer programs if you're
careful not to recurse forever, which is like an infin-
ite loop with more spectacular failure modes.
reference
Where you look to find a pointer to information some-
where else. (See "indirection".) References come in
two flavors, symbolic references and hard references.
referent
Whatever a reference refers to, which may or may not
have a name. Common types of referents include scalars,
arrays, hashes, and subroutines.
regex
See "regular expression".
regular expression
A single entity with various interpretations, like an
elephant. To a computer scientist, it's a grammar for a
little language in which some strings are legal and oth-
ers aren't. To normal people, it's a pattern you can
use to find what you're looking for when it varies from
case to case. Perl's regular expressions are far from
regular in the theoretical sense, but in regular use
they work quite well. Here's a regular expression: "/Oh
s.*t./". This will match strings like ""Oh say can you
see by the dawn's early light"" and ""Oh sit!"". See
perlre.
regular expression modifier
An option on a pattern or substitution, such as "/i" to
render the pattern case insensitive. See also "clois-
ter".
regular file
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A "file" that's not a "directory", a "device", a named
"pipe" or "socket", or a "symbolic link". Perl uses the
"-f" file test operator to identify regular files.
Sometimes called a "plain" file.
relational operator
An "operator" that says whether a particular ordering
relationship is "true" about a pair of operands. Perl
has both numeric and string relational operators. See
"collating sequence".
reserved words
A word with a specific, built-in meaning to a "com-
piler", such as "if" or delete. In many languages (not
Perl), it's illegal to use reserved words to name any-
thing else. (Which is why they're reserved, after all.)
In Perl, you just can't use them to name labels or
filehandles. Also called "keywords".
return value
The "value" produced by a "subroutine" or "expression"
when evaluated. In Perl, a return value may be either a
"list" or a "scalar".
RFC Request For Comment, which despite the timid connota-
tions is the name of a series of important standards
documents.
right shift
A "bit shift" that divides a number by some power of 2.
root
The superuser (UID == 0). Also, the top-level directory
of the filesystem.
RTFM
What you are told when someone thinks you should Read
The Fine Manual.
run phase
Any time after Perl starts running your main program.
See also "compile phase". Run phase is mostly spent in
"run time" but may also be spent in "compile time" when
require, do "FILE", or eval "STRING" operators are exe-
cuted or when a substitution uses the "/ee" modifier.
run time
The time when Perl is actually doing what your code says
to do, as opposed to the earlier period of time when it
was trying to figure out whether what you said made any
sense whatsoever, which is "compile time".
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run-time pattern
A pattern that contains one or more variables to be
interpolated before parsing the pattern as a "regular
expression", and that therefore cannot be analyzed at
compile time, but must be re-analyzed each time the pat-
tern match operator is evaluated. Run-time patterns are
useful but expensive.
RV A recreational vehicle, not to be confused with vehicu-
lar recreation. RV also means an internal Reference
Value of the type a "scalar" can hold. See also "IV"
and "NV" if you're not confused yet.
rvalue
A "value" that you might find on the right side of an
"assignment". See also "lvalue".
S
scalar
A simple, singular value; a number, "string", or "refer-
ence".
scalar context
The situation in which an "expression" is expected by
its surroundings (the code calling it) to return a sin-
gle "value" rather than a "list" of values. See also
"context" and "list context". A scalar context sometimes
imposes additional constraints on the return value--see
"string context" and "numeric context". Sometimes we
talk about a "Boolean context" inside conditionals, but
this imposes no additional constraints, since any scalar
value, whether numeric or "string", is already true or
false.
scalar literal
A number or quoted "string"--an actual "value" in the
text of your program, as opposed to a "variable".
scalar value
A value that happens to be a "scalar" as opposed to a
"list".
scalar variable
A "variable" prefixed with "$" that holds a single
value.
scope
How far away you can see a variable from, looking
through one. Perl has two visibility mechanisms: it
does "dynamic scoping" of local variables, meaning that
the rest of the "block", and any subroutines that are
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called by the rest of the block, can see the variables
that are local to the block. Perl does "lexical scop-
ing" of my variables, meaning that the rest of the block
can see the variable, but other subroutines called by
the block cannot see the variable.
scratchpad
The area in which a particular invocation of a particu-
lar file or subroutine keeps some of its temporary
values, including any lexically scoped variables.
script
A text "file" that is a program intended to be executed
directly rather than compiled to another form of file
before execution. Also, in the context of "Unicode", a
writing system for a particular language or group of
languages, such as Greek, Bengali, or Klingon.
script kiddie
A "cracker" who is not a "hacker", but knows just enough
to run canned scripts. A cargo-cult programmer.
sed A venerable Stream EDitor from which Perl derives some
of its ideas.
semaphore
A fancy kind of interlock that prevents multiple threads
or processes from using up the same resources simultane-
ously.
separator
A "character" or "string" that keeps two surrounding
strings from being confused with each other. The split
function works on separators. Not to be confused with
delimiters or terminators. The "or" in the previous
sentence separated the two alternatives.
serialization
Putting a fancy "data structure" into linear order so
that it can be stored as a "string" in a disk file or
database or sent through a "pipe". Also called marshal-
ling.
server
In networking, a "process" that either advertises a
"service" or just hangs around at a known location and
waits for clients who need service to get in touch with
it.
service
Something you do for someone else to make them happy,
like giving them the time of day (or of their life). On
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some machines, well-known services are listed by the
getservent function.
setgid
Same as "setuid", only having to do with giving away
"group" privileges.
setuid
Said of a program that runs with the privileges of its
"owner" rather than (as is usually the case) the
privileges of whoever is running it. Also describes the
bit in the mode word ("permission bits") that controls
the feature. This bit must be explicitly set by the
owner to enable this feature, and the program must be
carefully written not to give away more privileges than
it ought to.
shared memory
A piece of "memory" accessible by two different
processes who otherwise would not see each other's
memory.
shebang
Irish for the whole McGillicuddy. In Perl culture, a
portmanteau of "sharp" and "bang", meaning the "#!"
sequence that tells the system where to find the inter-
preter.
shell
A "command"-line "interpreter". The program that
interactively gives you a prompt, accepts one or more
lines of input, and executes the programs you mentioned,
feeding each of them their proper arguments and input
data. Shells can also execute scripts containing such
commands. Under Unix, typical shells include the Bourne
shell (/bin/sh), the C shell (/bin/csh), and the Korn
shell (/bin/ksh). Perl is not strictly a shell because
it's not interactive (although Perl programs can be
interactive).
side effects
Something extra that happens when you evaluate an
"expression". Nowadays it can refer to almost anything.
For example, evaluating a simple assignment statement
typically has the "side effect" of assigning a value to
a variable. (And you thought assigning the value was
your primary intent in the first place!) Likewise,
assigning a value to the special variable $| ($AUTO-
FLUSH) has the side effect of forcing a flush after
every write or print on the currently selected filehan-
dle.
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signal
A bolt out of the blue; that is, an event triggered by
the "operating system", probably when you're least
expecting it.
signal handler
A "subroutine" that, instead of being content to be
called in the normal fashion, sits around waiting for a
bolt out of the blue before it will deign to "execute".
Under Perl, bolts out of the blue are called signals,
and you send them with the kill built-in. See "%SIG" in
perlvar and "Signals" in perlipc.
single inheritance
The features you got from your mother, if she told you
that you don't have a father. (See also "inheritance"
and "multiple inheritance".) In computer languages, the
notion that classes reproduce asexually so that a given
class can only have one direct ancestor or "base class".
Perl supplies no such restriction, though you may cer-
tainly program Perl that way if you like.
slice
A selection of any number of elements from a "list",
"array", or "hash".
slurp
To read an entire "file" into a "string" in one opera-
tion.
socket
An endpoint for network communication among multiple
processes that works much like a telephone or a post
office box. The most important thing about a socket is
its "network address" (like a phone number). Different
kinds of sockets have different kinds of addresses--some
look like filenames, and some don't.
soft reference
See "symbolic reference".
source filter
A special kind of "module" that does "preprocessing" on
your script just before it gets to the "tokener".
stack
A device you can put things on the top of, and later
take them back off in the opposite order in which you
put them on. See "LIFO".
standard
Included in the official Perl distribution, as in a
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standard module, a standard tool, or a standard Perl
"manpage".
standard error
The default output "stream" for nasty remarks that don't
belong in "standard output". Represented within a Perl
program by the "filehandle" "STDERR". You can use this
stream explicitly, but the die and warn built-ins write
to your standard error stream automatically.
standard I/O
A standard C library for doing buffered input and output
to the "operating system". (The "standard" of standard
I/O is only marginally related to the "standard" of
standard input and output.) In general, Perl relies on
whatever implementation of standard I/O a given operat-
ing system supplies, so the buffering characteristics of
a Perl program on one machine may not exactly match
those on another machine. Normally this only influences
efficiency, not semantics. If your standard I/O package
is doing block buffering and you want it to "flush" the
buffer more often, just set the $| variable to a true
value.
standard input
The default input "stream" for your program, which if
possible shouldn't care where its data is coming from.
Represented within a Perl program by the "filehandle"
"STDIN".
standard output
The default output "stream" for your program, which if
possible shouldn't care where its data is going.
Represented within a Perl program by the "filehandle"
"STDOUT".
stat structure
A special internal spot in which Perl keeps the informa-
tion about the last "file" on which you requested infor-
mation.
statement
A "command" to the computer about what to do next, like
a step in a recipe: "Add marmalade to batter and mix
until mixed." A statement is distinguished from a
"declaration", which doesn't tell the computer to do
anything, but just to learn something.
statement modifier
A "conditional" or "loop" that you put after the "state-
ment" instead of before, if you know what we mean.
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static
Varying slowly compared to something else. (Unfor-
tunately, everything is relatively stable compared to
something else, except for certain elementary particles,
and we're not so sure about them.) In computers, where
things are supposed to vary rapidly, "static" has a
derogatory connotation, indicating a slightly dysfunc-
tional "variable", "subroutine", or "method". In Perl
culture, the word is politely avoided.
static method
No such thing. See "class method".
static scoping
No such thing. See "lexical scoping".
static variable
No such thing. Just use a "lexical variable" in a scope
larger than your "subroutine".
status
The "value" returned to the parent "process" when one of
its child processes dies. This value is placed in the
special variable $?. Its upper eight bits are the exit
status of the defunct process, and its lower eight bits
identify the signal (if any) that the process died from.
On Unix systems, this status value is the same as the
status word returned by wait(2). See "system" in perl-
func.
STDERR
See "standard error".
STDIN
See "standard input".
STDIO
See "standard I/O".
STDOUT
See "standard output".
stream
A flow of data into or out of a process as a steady
sequence of bytes or characters, without the appearance
of being broken up into packets. This is a kind of
"interface"--the underlying "implementation" may well
break your data up into separate packets for delivery,
but this is hidden from you.
string
A sequence of characters such as "He said !@#*&%@#*?!".
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A string does not have to be entirely printable.
string context
The situation in which an expression is expected by its
surroundings (the code calling it) to return a "string".
See also "context" and "numeric context".
stringification
The process of producing a "string" representation of an
abstract object.
struct
C keyword introducing a structure definition or name.
structure
See "data structure".
subclass
See "derived class".
subpattern
A component of a "regular expression" pattern.
subroutine
A named or otherwise accessible piece of program that
can be invoked from elsewhere in the program in order to
accomplish some sub-goal of the program. A subroutine
is often parameterized to accomplish different but
related things depending on its input arguments. If the
subroutine returns a meaningful "value", it is also
called a "function".
subscript
A "value" that indicates the position of a particular
"array" "element" in an array.
substitution
Changing parts of a string via the "s///" operator. (We
avoid use of this term to mean "variable interpola-
tion".)
substring
A portion of a "string", starting at a certain "charac-
ter" position ("offset") and proceeding for a certain
number of characters.
superclass
See "base class".
superuser
The person whom the "operating system" will let do
almost anything. Typically your system administrator or
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someone pretending to be your system administrator. On
Unix systems, the "root" user. On Windows systems, usu-
ally the Administrator user.
SV Short for "scalar value". But within the Perl inter-
preter every "referent" is treated as a member of a
class derived from SV, in an object-oriented sort of
way. Every "value" inside Perl is passed around as a C
language "SV*" pointer. The SV "struct" knows its own
"referent type", and the code is smart enough (we hope)
not to try to call a "hash" function on a "subroutine".
switch
An option you give on a command line to influence the
way your program works, usually introduced with a minus
sign. The word is also used as a nickname for a "switch
statement".
switch cluster
The combination of multiple command-line switches (e.g.,
-a -b -c) into one switch (e.g., -abc). Any switch with
an additional "argument" must be the last switch in a
cluster.
switch statement
A program technique that lets you evaluate an "expres-
sion" and then, based on the value of the expression, do
a multiway branch to the appropriate piece of code for
that value. Also called a "case structure", named after
the similar Pascal construct. Most switch statements in
Perl are spelled "for". See "Basic BLOCKs and Switch
Statements" in perlsyn.
symbol
Generally, any "token" or "metasymbol". Often used more
specifically to mean the sort of name you might find in
a "symbol table".
symbol table
Where a "compiler" remembers symbols. A program like
Perl must somehow remember all the names of all the
variables, filehandles, and subroutines you've used. It
does this by placing the names in a symbol table, which
is implemented in Perl using a "hash table". There is a
separate symbol table for each "package" to give each
package its own "namespace".
symbolic debugger
A program that lets you step through the execution of
your program, stopping or printing things out here and
there to see whether anything has gone wrong, and if so,
what. The "symbolic" part just means that you can talk
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to the debugger using the same symbols with which your
program is written.
symbolic link
An alternate filename that points to the real
"filename", which in turn points to the real "file".
Whenever the "operating system" is trying to parse a
"pathname" containing a symbolic link, it merely substi-
tutes the new name and continues parsing.
symbolic reference
A variable whose value is the name of another variable
or subroutine. By dereferencing the first variable, you
can get at the second one. Symbolic references are
illegal under use strict 'refs'.
synchronous
Programming in which the orderly sequence of events can
be determined; that is, when things happen one after the
other, not at the same time.
syntactic sugar
An alternative way of writing something more easily; a
shortcut.
syntax
From Greek, "with-arrangement". How things (particu-
larly symbols) are put together with each other.
syntax tree
An internal representation of your program wherein
lower-level constructs dangle off the higher-level con-
structs enclosing them.
syscall
A "function" call directly to the "operating system".
Many of the important subroutines and functions you use
aren't direct system calls, but are built up in one or
more layers above the system call level. In general,
Perl programmers don't need to worry about the distinc-
tion. However, if you do happen to know which Perl
functions are really syscalls, you can predict which of
these will set the $! ($ERRNO) variable on failure.
Unfortunately, beginning programmers often confusingly
employ the term "system call" to mean what happens when
you call the Perl system function, which actually
involves many syscalls. To avoid any confusion, we
nearly always use say "syscall" for something you could
call indirectly via Perl's syscall function, and never
for something you would call with Perl's system func-
tion.
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T
tainted
Said of data derived from the grubby hands of a user and
thus unsafe for a secure program to rely on. Perl does
taint checks if you run a "setuid" (or "setgid") pro-
gram, or if you use the -T switch.
TCP Short for Transmission Control Protocol. A protocol
wrapped around the Internet Protocol to make an unreli-
able packet transmission mechanism appear to the appli-
cation program to be a reliable "stream" of bytes.
(Usually.)
term
Short for a "terminal", that is, a leaf node of a "syn-
tax tree". A thing that functions grammatically as an
"operand" for the operators in an expression.
terminator
A "character" or "string" that marks the end of another
string. The $/ variable contains the string that ter-
minates a readline operation, which chomp deletes from
the end. Not to be confused with delimiters or separa-
tors. The period at the end of this sentence is a ter-
minator.
ternary
An "operator" taking three operands. Sometimes pro-
nounced "trinary".
text
A "string" or "file" containing primarily printable
characters.
thread
Like a forked process, but without "fork"'s inherent
memory protection. A thread is lighter weight than a
full process, in that a process could have multiple
threads running around in it, all fighting over the same
process's memory space unless steps are taken to protect
threads from each other. See threads.
tie The bond between a magical variable and its implementa-
tion class. See "tie" in perlfunc and perltie.
TMTOWTDI
There's More Than One Way To Do It, the Perl Motto. The
notion that there can be more than one valid path to
solving a programming problem in context. (This doesn't
mean that more ways are always better or that all possi-
ble paths are equally desirable--just that there need
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not be One True Way.) Pronounced TimToady.
token
A morpheme in a programming language, the smallest unit
of text with semantic significance.
tokener
A module that breaks a program text into a sequence of
tokens for later analysis by a parser.
tokenizing
Splitting up a program text into tokens. Also known as
"lexing", in which case you get "lexemes" instead of
tokens.
toolbox approach
The notion that, with a complete set of simple tools
that work well together, you can build almost anything
you want. Which is fine if you're assembling a tricy-
cle, but if you're building a defranishizing comboflux
regurgalator, you really want your own machine shop in
which to build special tools. Perl is sort of a machine
shop.
transliterate
To turn one string representation into another by map-
ping each character of the source string to its
corresponding character in the result string. See
"tr/SEARCHLIST/REPLACEMENTLIST/cds" in perlop.
trigger
An event that causes a "handler" to be run.
trinary
Not a stellar system with three stars, but an "operator"
taking three operands. Sometimes pronounced "ternary".
troff
A venerable typesetting language from which Perl derives
the name of its $% variable and which is secretly used
in the production of Camel books.
true
Any scalar value that doesn't evaluate to 0 or "".
truncating
Emptying a file of existing contents, either automati-
cally when opening a file for writing or explicitly via
the truncate function.
type
See "data type" and "class".
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type casting
Converting data from one type to another. C permits
this. Perl does not need it. Nor want it.
typed lexical
A "lexical variable" that is declared with a "class"
type: "my Pony $bill".
typedef
A type definition in the C language.
typeglob
Use of a single identifier, prefixed with "*". For
example, *name stands for any or all of $name, @name,
%name, &name, or just "name". How you use it determines
whether it is interpreted as all or only one of them.
See "Typeglobs and Filehandles" in perldata.
typemap
A description of how C types may be transformed to and
from Perl types within an "extension" module written in
"XS".
U
UDP User Datagram Protocol, the typical way to send
datagrams over the Internet.
UID A user ID. Often used in the context of "file" or "pro-
cess" ownership.
umask
A mask of those "permission bits" that should be forced
off when creating files or directories, in order to
establish a policy of whom you'll ordinarily deny access
to. See the umask function.
unary operator
An operator with only one "operand", like "!" or chdir.
Unary operators are usually prefix operators; that is,
they precede their operand. The "++" and "--" operators
can be either prefix or postfix. (Their position does
change their meanings.)
Unicode
A character set comprising all the major character sets
of the world, more or less. See
<http://www.unicode.org>.
Unix
A very large and constantly evolving language with
several alternative and largely incompatible syntaxes,
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in which anyone can define anything any way they choose,
and usually do. Speakers of this language think it's
easy to learn because it's so easily twisted to one's
own ends, but dialectical differences make tribal inter-
communication nearly impossible, and travelers are often
reduced to a pidgin-like subset of the language. To be
universally understood, a Unix shell programmer must
spend years of study in the art. Many have abandoned
this discipline and now communicate via an Esperanto-
like language called Perl.
In ancient times, Unix was also used to refer to some
code that a couple of people at Bell Labs wrote to make
use of a PDP-7 computer that wasn't doing much of any-
thing else at the time.
V
value
An actual piece of data, in contrast to all the vari-
ables, references, keys, indexes, operators, and whatnot
that you need to access the value.
variable
A named storage location that can hold any of various
kinds of "value", as your program sees fit.
variable interpolation
The "interpolation" of a scalar or array variable into a
string.
variadic
Said of a "function" that happily receives an indeter-
minate number of "actual arguments".
vector
Mathematical jargon for a list of scalar values.
virtual
Providing the appearance of something without the real-
ity, as in: virtual memory is not real memory. (See
also "memory".) The opposite of "virtual" is "tran-
sparent", which means providing the reality of something
without the appearance, as in: Perl handles the
variable-length UTF-8 character encoding transparently.
void context
A form of "scalar context" in which an "expression" is
not expected to return any "value" at all and is
evaluated for its "side effects" alone.
v-string
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A "version" or "vector" "string" specified with a "v"
followed by a series of decimal integers in dot nota-
tion, for instance, "v1.20.300.4000". Each number turns
into a "character" with the specified ordinal value.
(The "v" is optional when there are at least three
integers.)
W
warning
A message printed to the "STDERR" stream to the effect
that something might be wrong but isn't worth blowing up
over. See "warn" in perlfunc and the warnings pragma.
watch expression
An expression which, when its value changes, causes a
breakpoint in the Perl debugger.
whitespace
A "character" that moves your cursor but doesn't other-
wise put anything on your screen. Typically refers to
any of: space, tab, line feed, carriage return, or form
feed.
word
In normal "computerese", the piece of data of the size
most efficiently handled by your computer, typically 32
bits or so, give or take a few powers of 2. In Perl
culture, it more often refers to an alphanumeric "iden-
tifier" (including underscores), or to a string of
nonwhitespace characters bounded by whitespace or string
boundaries.
working directory
Your current "directory", from which relative pathnames
are interpreted by the "operating system". The operat-
ing system knows your current directory because you told
it with a chdir or because you started out in the place
where your parent "process" was when you were born.
wrapper
A program or subroutine that runs some other program or
subroutine for you, modifying some of its input or out-
put to better suit your purposes.
WYSIWYG
What You See Is What You Get. Usually used when some-
thing that appears on the screen matches how it will
eventually look, like Perl's format declarations. Also
used to mean the opposite of magic because everything
works exactly as it appears, as in the three-argument
form of open.
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X
XS An extraordinarily exported, expeditiously excellent,
expressly eXternal Subroutine, executed in existing C or
C++ or in an exciting new extension language called
(exasperatingly) XS. Examine perlxs for the exact
explanation or perlxstut for an exemplary unexacting
one.
XSUB
An external "subroutine" defined in "XS".
Y
yacc
Yet Another Compiler Compiler. A parser generator
without which Perl probably would not have existed. See
the file perly.y in the Perl source distribution.
Z
zero width
A subpattern "assertion" matching the "null string"
between characters.
zombie
A process that has died (exited) but whose parent has
not yet received proper notification of its demise by
virtue of having called wait or waitpid. If you fork,
you must clean up after your child processes when they
exit, or else the process table will fill up and your
system administrator will Not Be Happy with you.
Based on the Glossary of Programming Perl, Third Edition, by
Larry Wall, Tom Christiansen & Jon Orwant. Copyright (c)
2000, 1996, 1991 O'Reilly Media, Inc. This document may be
distributed under the same terms as Perl itself.
perl v5.8.8 2006-06-30 58